General
On this page 4 sections
Development of Head
| Primary Vesicle | Secondary Vesicle | Derivatives |
| Prosencephalon (Forebrain) | Telencephalon | Cerebral cortex, basal ganglia, hippocampus, amygdala, olfactory bulb |
| Diencephalon | Thalamus, hypothalamus, epithalamus (pineal gland), subthalamus, retina, optic nerve | |
| Mesencephalon (Midbrain) | Mesencephalon | Midbrain (tectum, cerebral peduncles, superior & inferior colliculi) |
| Rhombencephalon (Hindbrain) | Metencephalon | Cerebellum, pons |
| Myelencephalon | Medulla oblongata |
Trimesters Overview
| Embryonic / Fetal Age | Major Developmental Milestones & Features |
| 1st Month (Weeks 1–4) | * Week 1: Fertilization, cleavage, blastocyst formation, implantation. * Week 2: Bilaminar germ disc (epiblast & hypoblast), amniotic cavity, primitive yolk sac. * Week 3: Gastrulation (3 germ layers), primitive streak, neurulation begins, heart starts beating. * Week 4: Pharyngeal arches, neural tube closes, upper/lower limb buds appear. |
| 2nd Month (Weeks 5–8) | * Hand and foot plates form, fingers and toes separate. * External genitalia begin to differentiate (indifferent stage at start). * Face forms (fusion of facial prominences). * End of Week 8: Organogenesis is largely complete; embryo transitions to a fetus. |
| 3rd Month (Weeks 9–12) | * Urine production begins. * Primary ossification centers appear in long bones and skull. * External genitalia become clearly distinguishable (Week 12). * Intestines return to the abdominal cavity from the umbilical cord (midgut herniation resolves). |
| 4th Month (Weeks 13–16) | * Myelination of the central and peripheral nervous systems begins. * Lanugo (fine hair) and vernix caseosa start to form. * Fetal movements begin (though usually not felt by the mother yet). * Eyes move to the front of the face, ears reach final position. |
| 5th Month (Weeks 17–20) | * Quickening (first fetal movements felt by the mother). * Brown fat forms for heat production. * Hair on the head (eyebrows/eyelashes) becomes visible. |
| 6th Month (Weeks 21–24) | * Type II pneumocytes start secreting surfactant (around Week 24). * Fingernails are present. * Fetus gains significant weight; skin is reddish and wrinkled. |
| 7th Month (Weeks 25–28) | * Eyes open; light responsiveness develops. * Bone marrow takes over as the primary site of hematopoiesis. |
| 8th & 9th Months (Weeks 29–38) | * Testes descend into the scrotum (Weeks 28–32). * Deposition of subcutaneous fat rounds out the body. * Surfactant levels reach maturity for breathing air at term. |
Derivatives
| Germ Layer / Structure | Key Neural Crest & Mesoderm Derivatives (High-Yield MCQs) |
| Neural Crest Cells | * Adrenal medulla (chromaffin cells) * Melanocytes (pigment cells) * Schwann cells & glial cells * Sensory & autonomic ganglia (dorsal root, sympathetic chain) * Cranial facial bones, cartilage & connective tissue * PNS structures & Odontoblasts (dentine formation) * Conotruncal septum of heart |
| Surface Ectoderm | * Epidermis, hair, nails, sweat/sebaceous glands * Anterior pituitary (Rathke's pouch) * Lens & corneal epithelium of eye * Inner ear (otocysts) & lacrimal glands * Enamel of teeth |
| Neuroectoderm | * Central Nervous System (brain, spinal cord, astrocytes, oligodendrocytes) * Posterior pituitary (neurohypophysis) * Retina, optic nerve, and iris dilator/sphincter muscles |
| Paraxial Mesoderm (Somites) | * Axial skeleton (vertebrae, ribs, occipital bones via sclerotome) * Skeletal muscles (trunk and limb muscles via myotome) * Dermis of back skin (via dermatome) |
| Intermediate Mesoderm | * Kidneys & ureters * Gonads (testes/ovaries) & internal reproductive ducts |
| Lateral Plate Mesoderm | * Somatic: Parietal membranes, dermis of body wall, limb bones/cartilage * Splanchnic: Visceral layer, heart, blood vessels, smooth muscle & CT of GI tract |
| Endoderm | * Epithelial lining of GI tract, respiratory tract, urinary bladder, urethra * Parenchyma of liver, pancreas, thyroid, and parathyroid glands |
Development of structures
| Developmental Stage / Week | Structures Formed |
| 4th Week | Thyroid gland (originates as an endodermal proliferation at the foramen cecum) |
| 6th Week | Parotid gland, Submandibular gland |
| 7th Week | Muscles of mastication |
| 8th Week | Maxilla, Premaxilla, Sublingual gland |